Modern wireless communications systems are typically placed under potentially conflicting demands for high data capacity and high signal integrity. In order to make the best use of signal propagation conditions in terms of signal to noise ratio and signal dispersion, adaptive modulation systems may be deployed, in which a modulation mode is selected for use in a given channel on the basis of signal propagation conditions in that channel. A modulation mode may typically be selected that is expected to give the highest throughput consistent with an acceptable bit error rate.
It is well known that wireless communications systems, such as point-to-point microwave links between terminals, may use single carrier modulation schemes, in which adaptive modulation is used to select a single carrier modulation mode, such as a level of quadrature amplitude modulation (QAM), for example 16 QAM, 32 QAM or 64 QAM, according to received signal quality. Also, it is well known that wireless communications systems may use Orthogonal Frequency Division Multiplexing (OFDM), in which a number of sub-carriers are modulated, typically each subcarrier being modulated with a level of QAM modulation. In OFDM systems, adaptive modulation may be used to select an OFDM modulation mode, such as a level of quadrature amplitude modulation (QAM) on each subcarrier, and/or an arrangement of pilot tones and/or cyclic prefix length, according to received signal quality. Received signal quality may change, even in a system in which the position of the terminals is fixed, due to many factors, such as weather conditions and movement of obstructions, for example. Furthermore, interference levels may change with time.
A measure of received signal quality on a channel, such as signal to noise ratio, may be used to select a modulation mode. Before moving to a different modulation mode, it is advantageous to estimate whether or not the error rate performance in the modulation mode to be selected will be acceptable, and only to select the mode if the error rate performance is expected to be acceptable, as described in U.S. Pat. No. 7,469,013. This estimate may be on the basis of a measure of a vector error taken in the current modulation mode, a vector error being a difference between a received signal vector and an expected modulation state, such as a point on a QAM grid, that the received signal vector is expected to represent. However, it may be the case that it is desired to move from one mode of operation using a first type of equaliser to another mode of operation using a different type of equaliser, for example from an OFDM mode of operation to a single carrier mode of operation. In this case, one mode of operation may not provide a good indicator of the performance of the other mode of operation, for example the vector error of the OFDM mode may not be a good indicator of an error rate performance in the single carrier modulation mode to be selected. Therefore, it may not be possible to ensure that a transition from one mode to another, for example from an OFDM mode to a single carrier mode, will be “hitless”, i.e. without causing disruption at the transition and without reversion to the previous mode because the selected mode does not operate at an acceptable error rate.
It is an object of the invention to mitigate the problems of the prior art.